Refrigerating systems are commonly designed to include a compressor as well as a controller for determining the current cooling requirement of a cooling site. Whenever the controller identifies an increased cooling requirement, the controller notifies the compressor to increase performance. Refrigerating systems with a compressor equipped with a frequency converter also contain, in addition to the compressor and the controller, a frequency converter capable of changing the revolutions of the compressor motor in the range of an upper and lower number of revolutions per minute. Whenever the controller identifies an increased cooling requirement, the controller notifies the compressor to increase performance by adjusting its revolutions per minute and therefore the cooling liquid flow.
In addition, it is known that the refrigerating system should be equipped with a motor protection circuit in order to, at a minimum, monitor the parameter characterising compressor operations, such as the winding temperature of the motor, with the compressor switching off if a defined temperature threshold is exceeded or underrun. Such a motor protection circuit is e.g. set out in DE 10 2005 052 042 A1. Triggering motor protection however means that the cooling cycle is necessarily interrupted, which can have serious consequences. In addition, a(n automatic or manual) restart can involve uncertainty about what caused the trigger in the first place. Some compressor manufacturers only allow a compressor to restart in such a case if the cause has been identified and rectified. Such switching off is particularly annoying where no actual component defect or functional error occurred in the system and it switched off only because of an increased demand at the cooling site.
DE 10 2005 052 042 A1 therefore suggested that the current operating condition of the compressor should be taken into account when controlling the performance of the compressor. This can help to prevent a further performance increase from being initiated even though the compressor is approaching the switch-off threshold.
DE 10 2004 048 940 A1 also sets out a process for regulating the performance of a compressor for a refrigerating system where the compressor has a pneumatic or hydraulic servo mechanism for intermittent pauses in cooling liquid supply into a suction chamber. In addition, the compressor has a controller which can be used to create a pulse-width-modulated switch signal for the pneumatic or hydraulic servo mechanism to regulate the intermittent pauses in the cooling liquid supply. The on-off ratio for activating the pneumatic or hydraulic servo mechanism can be adapted to the needs of the cooling site.
DE 699 28 055 T2 and US 20090205349 A1 mention compressors, which regulate cooling liquid flow using a pulse-width-modulated switch signal. U.S. Pat. No. 6,925,823 B2 suggests that the compressor might run at reduced load when a first system state is reached and switched off only when a second system state is reached, in order to delay the switch-off point. DE 100 64 218 A1 describes a process for regulating a cooling unit with at least one compressor and at least two separate cold-storage rooms for different temperatures.
EP 1 710 435 B1 describes how to control activation of the valves on the input side of the compressor, which creates an opening or closing interval. Together, the two intervals create a switch interval. The switch interval must be shorter than the shortest time period during which the temperature in the evaporator of the refrigerating system increases by 10% without suction flow being disrupted.
However, the clocked opening and closing of the valve has the disadvantage that the cooling liquid flow, which normally flows through the motor to cool it, is reduced accordingly. This worsens motor cooling in the compressor, which makes the risk that the motor switches off due to the motor protection circuit more probable.